HSV-1 as a Model for Emerging Gene Delivery Vehicles

The majority of viral vectors currently used possess modest cargo capability (up to 40？kb) being based on retroviruses, lentiviruses, adenoviruses, and adenoassociated viruses. These vectors have made the most rapid transition from laboratory to clinic because their small genomes have simplified their characterization and modification. However, there is now an increasing need both in research and therapy to complement this repertoire with larger capacity vectors able to deliver multiple transgenes or to encode complex regulatory regions, constructs which can easily span more than 100？kb. Herpes Simplex Virus Type I (HSV-1) is a well-characterized human virus which is able to package about 150？kb of DNA, and several vector systems are currently in development for gene transfer applications, particularly in neurons where other systems have low efficiency. However, to reach the same level of versatility and ease of use as that of smaller genome viral vectors, simple systems for high-titer production must be developed. This paper reviews the major HSV-1 vector systems and analyses the common elements which may be most important to manipulate to achieve this goal. 1. Introduction Although horizontal gene transfer has long been recognized as an important factor in microbial evolution (reviewed in [1–3]) it is only recently that it has been recognized as a widespread phenomenon also in multicellular eukaryotes [4–6]. Among the multitude of mechanisms which have been identified, viruses have played and continue to play one of the major roles in horizontal gene transfer, leading to the concept of evolutionary units not being only cellular genomes but rather the sum of these plus their associated viruses [7, 8]. Many different classes of viruses appear to have been implicated in the lateral transfer of transposable elements during eukaryotic evolution [4, 6, 9–11] and together with the fact that a large variety of virus types continue to infect our cells, it is clear that a myriad of molecular mechanisms exist for the genetic modification of mammalian tissues. Taking example from the exquisite specificity and efficiency of viruses to change the phenotype of the cells which they infect, attempts to make gene transfer tools have relied heavily on viral vectors (reviewed in [12–16]). This field of “vectorology” has followed a synthetic biology approach, using both natural and engineered components to construct gene delivery vehicles tailored to specific research or therapeutic aims. Understandably, most eukaryotic viral vectors currently being used are those derived